1. Field of the Invention
The present invention relates to silicone rubber compositions. More particularly. this invention relates to the use of certain platinum-alkenylsiloxane catalysts in storage stable liquid silicone rubber compositions containing a reinforcing silica filler and an organopolysiloxane having at least two alkenyl radicals per molecule. The compositions are combined with an organohydrogensiloxane to yield curable liquid silicone rubber compositions.
2. Description of the Prior Art
Silicone rubber moldings have been produced by the thermal curing of compositions consisting of an organopolysiloxane gum, a reinforcing silica filler and an organoperoxide. However, this method requires complex processing before molding, such as mastication and portion-wise discharge. To avoid these complications, a method has been developed for producing silicone rubber moldings using liquid silicone rubber composition that are cured by heating.
A preferred type of liquid silicone rubber composition comprises an organopolysiloxane having silicon-bonded vinyl radicals and a relatively low degree of polymerization, an organohydrogenpolysiloxane and a reinforcing silica filler. These compositions typically include a curing catalyst in the form of a platinum-alkenylsiloxane complex to accelerate the addition reaction between the silicon-bonded vinyl radicals and silicon-bonded hydrogen atoms. This method offers numerous advantages, including ease of handling and the capability of being processed by injection molding with its excellent productivity. This method's range of application has as a consequence undergone steady expansion.
Liquid silicone rubber compositions typically cannot be stored as a one-package composition for extended periods of time because the addition reaction responsible for curing of the composition occurs even at room temperature. These type of compositions are usually made available in two packages. One of the packages comprises the mixture of the organopolysiloxane having silicon-bonded vinyl radicals, the reinforcing silica and platinum-alkenylsiloxane complex. The second of the two packages contains a crosslinker composition comprising a mixture of the organopolysiloxane having silicon-bonded hydrogen atoms and reinforcing silica. Immediately prior to molding, the contents of the two containers are mixed to homogeneity to yield a curable liquid silicone rubber composition which is then shaped and thermally cured to provide the molding. Packaging the liquid silicone rubber composition in two containers does not provide long-term storage stability for the composition comprising the organopolysiloxane having silicon-bonded vinyl radicals, the reinforcing silica and the platinum-alkenylsiloxane complex. This mixture may undergo viscosity changes during long-term storage. The curing behavior during molding of the silicone rubber may also be changed. In the extreme case, the platinum-alkenylsiloxane complex suffers a decline in its catalytic activity, and it is not possible to obtain excellent properties in the cured silicone rubber.
The present inventor carried out a vigorous investigation with a view to eliminating the above problems, and the present invention was developed as a result.
One objective of the present invention is to provide a storage-stable platinum-alkenylsiloxane catalyst composition. A second objective is to provide liquid silicone rubber compositions that contain these storage-stable catalysts and cure by a platinum-catalyzed hydrosilation reaction.